1. Field of the Invention
The present invention relates to a semiconductor device including a Schottky electrode made of nitride semiconductor material and a method for manufacturing the same.
2. Description of Related Art
Group III-V nitride semiconductors containing indium (In), aluminum (Al) and gallium (Ga) and represented by the general formula: InxAlyGa1-x-yN (0≦x, y≦1, x+y≦1) have unique characteristics which are not shown by conventionally used Si and GaAs, such as high breakdown electric field and high saturated electron velocity. At an interface between typical AlGaN/GaN hetero junction, two dimensional electron gas (2DEG) in as extremely high concentration as 1×1013 cm−2 is generated by the effects of spontaneous polarization and piezopolarization. By making full use of the characteristics, electron devices such as field effect transistors (FET) and Schottky barrier diodes (SBD) using the nitride semiconductors have actively been developed in recent years.
For the development of the devices using the group III-V nitride semiconductors, improvement in dielectric strength and reduction in leakage current are particularly important. GaN-based materials have high breakdown electric field. However, in a device such as FETs and SBDs, electric field concentration occurs in a particular location to cause breakdown in the device at a voltage extremely lower than the breakdown electric field. Further, the GaN-based materials are likely to produce deep surface states, through which leakage current flows between the electrodes.
For the purpose of reducing the electric field concentration and improving the dielectric strength, there is a known technique of providing a gate electrode of an FET with a field plate (e.g., see Japanese Unexamined Patent Publication No. 2004-200248 and “Electronic Letters”, 2001, Vol. 37, No. 3, pp. 196-197).
Hereinafter, an explanation of the conventional FET including the field plate will be provided with reference to the drawings. FIG. 10 shows the cross-sectional structure of the conventional FET. As shown in FIG. 10, a 1 μm thick undoped gallium nitride (GaN) layer 53 and a 25 nm thick n-doped aluminum gallium nitride (AlGaN) layer 54 are formed in this order on a sapphire substrate 5 with a buffer layer 52 made of aluminum nitride (AlN) sandwiched therebetween. A source electrode 56 and a drain electrode 57 serving as ohmic electrodes are formed on the AlGaN layer 54 to be spaced from each other. A gate electrode 58 is formed as a Schottky electrode between the source electrode 56 and the drain electrode 57. The gate electrode 58 is configured to extend over a SiN film 59 toward the drain electrode 57.
When a high voltage is applied to the drain electrode 57, in general, a highest electric field is applied to an end 58b of the gate electrode 58 facing the drain electrode 57. In this FET, part of the gate electrode 58 extending over the SiN film 59 toward the drain electrode 57 in the eave-like form serves as a field plate 58a. Therefore, the electric field generated between the gate electrode 58 and the drain electrode 57 is also distributed in the field plate 58a. Therefore, the field intensity at the end 58b of the gate electrode 58 facing the drain electrode 57 is reduced, thereby improving the dielectric strength of the device.
Also in the SBD, similar electric field concentration occurs between the Schottky electrode and the ohmic electrode. Therefore, the Schottky electrode is configured to have a field plate extending toward the ohmic electrode, thereby improving the dielectric strength.
As the surface of the AlGaN layer 54 is covered with the SiN film 59, leakage current is reduced as compared with the case where the AlGaN layer 54 is exposed to air.
The conventional technique improves the dielectric strength effectively as described above. However, the technique is hardly effective in reducing leakage current which occurs on the application of a reverse bias.
In a III-V nitride semiconductor device, a surface layer is generally made of AlGaN which is a mixed crystal of Ga and Al and has a relatively small band gap. AlGaN has many crystal defects on the surface thereof and they produce deep surface states. Therefore, when a reverse bias is applied, leakage current flows between the electrodes through the surface states. Even if the surface of the AlGaN layer is covered with a SiN film, the surface state density hardly decreases. Therefore, it is almost impossible to reduce the leakage current.